The world's most important cereal grains—rice, wheat, barley, and corn—are all crops whose seeds (embryo and endosperm) are used as food. In barley, each rachis forms three monanthous spikelets. Those in which all three spikelets bear seeds are called six-rowed barleys; those in which only the center spikelet bears seeds are called two-rowed barleys. Although two-rowed and six-rowed barleys are the same biological species, their origins and history are different, and the plants have many different morphological, physiological, and ecological traits, which contributes to their different qualities and uses. It is believed that six-rowed barley first came to Japan from Eurasia around the first century. Six-rowed barley has been long used as a food to supplement rice, and has also been used as livestock feed. It is also used as an ingredient for making miso and soy sauce. In contrast, two-rowed barley was introduced from Europe during the Meiji era or later. Two-rowed barley is characterized by a low protein content and high starch ratio. It has superior uniformity in the malting process, and is thus mainly used for beer brewing.
Differences in barley row type are known to be controlled by a single gene (vrs1), which is localized on chromosome 2. Detailed comparisons of the morphologies of two-rowed and six-rowed barleys show a number of notable changes in two-rowed barley, such as a size reduction in the two lateral spikelets of the three spikelets, regression of stamens, trailed pistils, and disappearance of aristae, revealing pleiotropic expression of a single gene. In addition, agronomically important traits such as flowering time and plant height, and a variety of brewing characteristics, are linked to the genomic region containing the gene that controls row type. Thus, the genomic region is particularly important. Moreover, it was recently found that the gene for resistance to Fusarium head blight in barley and related Triticeae plants (quantitative trait loci: QTL) is closely linked with the gene that controls row type in barley (de la Pena et al. Theor. Appl. Genet. 99: 561-569 (1999); Zhu et al. Theor. Appl. Genet. 99: 1221-1232 (1999)).
Fusarium head blight (FHB) in barley and related Triticeae plants is a serious disease that contaminates many Gramineae crops such as wheat, barley, and oats, not only reducing the commercial values of these grains, but also producing mycotoxins such as deoxynivalenol. Deoxynivalenol is an extremely dangerous toxin, causing gastrointestinal disorders accompanied by hemorrhagic conditions and the like in humans and animals that eat infected grains, leading to death in some cases. Since deoxynivalenol is stable against changes in pH and heat, detoxification is difficult. Therefore, grains contaminated beyond a certain level cannot be used in any form of brewing, processing, or livestock feed, and are thus disposed of. The pathogen Fusarium spp. is a very common saprobe, spread across grain cultivating areas all over the world, and known to cause severe damage, particularly in areas with high rainfall between flowering and grain filling. On the other hand, increased concerns about food safety have led to requests for cultivation using as little pesticides as possible. Thus, development of disease-resistant cultivars is essential in improving the safety of barley and related Triticeae plants. These problems require urgent solution not only in Asia, but also across the globe, including the United States and Europe.
However, progress in improving barley resistance to Fusarium head blight has been slow. The reasons for this are as follows: The small number of resistant barley cultivars also carry a number of agronomically unfavorable traits, and can not be effectively used as breeding materials. In addition, the resistance trait can only be identified during maturation stages, making it impossible to use a marker to carry out early selection for resistance. Moreover, designing appropriate breeding strategies has been difficult, since it was not clear whether resistance is controlled by the same gene that controls row type, or whether the two genes are closely linked with each other and difficult to separate. Thus, to solve these problems, the development of molecular markers and establishment of identification methods using earlier generations has been desired.